The present invention generally relates to ignition systems and more particularly to such systems, as well as to an exciter circuit and a method of igniting fuel.
Ignition systems for turbine engines as well as other applications have been in use for decades and they continue to evolve with changing technology. Recent developments have included the incorporation and use of solid state semiconductor power switching devices for releasing energy from an energy storage device for generating a spark discharge for igniting fuel in a turbine engine, for example. Such solid state devices are considered to be more reliable than gas discharge tubes that had been previously employed for decades. Because such systems often have to reliably operate in severe environmental conditions that include significant temperature and air pressure variations, and because reliability and safety considerations are of paramount concern when the ignition systems are used in aircraft engines, for example, such systems must be carefully designed for effective and reliable operation.
It has been generally consistent practice to design exciter circuitry that is used in connection with an igniter plug to employ a relatively high voltage bus, i.e., on the order of at least 2000 to 3000 volts, so that the igniter plug reliably produces a sufficient spark during operation. Serious design consideration has been given to not only producing a sufficient initial spark, but also one that is sustained so that reliable ignition of the fuel occurs in the engine, particularly in severe environmental conditions. However, when a high voltage bus is utilized in the design of the exciter circuit, the components that operate in the circuit must be capable of withstanding the high voltage and current loads that are experienced. For example, if a high energy capacitor is utilized in an exciter circuit and its energy is released by a silicon controlled rectifier (SCR) switch, such a single SCR switch that can handle the high voltage and current loading may be very expensive. Alternatively, a switch design may be utilized which employs multiple SCR""s connected in a more complex circuit arrangement. More particularly, such high voltage switching is often performed by multiple series connected SCR""s which must be very carefully matched and triggered during operation or they will likely prematurely fail.
While such high voltage ignition systems not only experience the problems associated with finding reliable and cost efficient components that can be used in such a high voltage environment, they also do not necessarily result in the most efficient ignition current waveform of energy delivery to the igniter plug. Typically, a wave shaping inductor is placed between the energy storage capacitor and the igniter in order to increase the current duration and decrease the peak current going to the igniter.
The present invention includes a preferred embodiment ignition system for a turbine engine which includes an exciter circuit that has a step-up transformer utilizing a relatively low voltage in its primary to produce a high voltage pulse that is applied to an igniter to create ionization and breakdown. The system also utilizes a low voltage high energy circuit to provide high current energy to the igniter after initial breakdown and during the plasma arc phase. The high energy circuit is decoupled from the step-up transformer so that high current is conducted through a bypass rather than through the transformer. Moreover, the low voltage of the high energy circuit allows for smaller, less expensive and more robust semiconductors to be used as the high energy switch.
The exciter circuitry carefully times the release of energy from a separate primary side capacitor to the step-up transformer relative to the operation of the SCR switch that releases the energy from the high energy capacitor, which desirably protects the high energy SCR switch during generation of the high voltage pulse that is applied to the igniter plug. The low voltage topology, which utilizes very large capacitance for the high energy capacitors, produces an ignition current waveform with longer duration and lower peak current than traditional prior art systems of equivalent stored energy. The lower peak currents place lower peak power stresses on the exciter components, while the longer duration ensures high energy delivery through the igniter plug to the combustible air/fuel mixture.
In the preferred embodiment of the present invention, the high capacitance (e.g., 75 xcexcF) associated with the low voltage system (e.g., 650V) allows for increasing current durations in the presence of increasing external resistance. The low capacitance (e.g., 3.5 xcexcF) associated with a traditional high voltage (e.g., 2800V) system typically requires the addition of a current discharge wave shaping inductor which increases the current duration while reducing the peak currents to reasonable levels. Furthermore, a low capacitance, unipolar system utilizing a typical wave shaping inductor exhibits decreasing current durations in the presence of increasing external resistance. Thus, the energy delivery in the presence of increasing external resistance is more consistent with a low voltage system. Sources of external resistance include the ignition lead, which connects the exciter and igniter, along with the igniter and igniter extensions.